Regulation of lactic acid production by Leptotrichia buccalis

Two lactate dehydrogenase (EC 1.1.1.27) species were separated by agarose A 0.5 M molecular exclusion column chromatography from cell-free extracts obtained from L. buccalis. Both lactate dehydrogenases were under negative control by ATP but the kinetic responses of the respective lactate dehydrogenase species to ATP were separate and distinct. The higher MW lactate dehydrogenase (LDH1) showed a negative hyperbolic kinetic response to ATP and Dixon plots of the ATP saturation data gave a Ki for ATP of 1.8 M. The activity of the lower MW lactate dehydrogenase (LDH2) was a negative sigmoidal function of the ATP concentration and had an [M]0.5V value for ATP of 2.5 M. A Hill coefficient of 1.95 was obtained from Hill plots of the ATP saturation data for LDH2, which indicated multiple ATP binding sites on the enzyme with some degree of cooperation interaction between them.